Matti Tomi is working as a research assistant at TKK's Low Temperature Laboratory. Along with wrapping up his M.Sc. studies at TKK he's doing research on the electrical properties of carbon nanotubes and graphene. His current work on the characterization of graphene sheets will also lay the foundation for an upcoming diploma thesis on low-temperature high-frequency measurements on graphene. "In the past we have carried out transport and noise measurements on graphene samples obtained from collaborators at Delft University of Technology in the Netherlands. More recently, we have put more weight on optimizing our own sample manufacturing procedures to provide for more sophisticated and complex experiments."

The Nano group of the Low Temperature Laboratory (LTL) consists of some 15 scientists investigating quantum phenomena in nanostructures such as Josephson junctions, carbon nanotubes and graphene. Thanks to the long-running experience in cryoengineering at LTL, most experiments can be carried out at temperatures ranging from that of liquid helium (4 K at 1 atm) down to the millikelvin temperatures of dilution cryostats. In graphene-related research LTL operates in collaboration with several TKK laboratories and Nokia Research Center.

Although practically all the measurements are performed at LTL, in the preparatory work Tomi frequently finds himself moving about in the campus area. "It's often all about finding the right tool for the job. And this is where the cooperation among UMK members really pays off. In addition to our own facilities, within the graphene project we've used the equipment at Micronova, at the Forest Products Chemistry lab and at the Laboratory of Physics, all within a walking distance."

After decades of being shrugged off as a merely theoretical model, the experimental discovery of graphene in 2004 opened up a whole new research area in materials science and mesoscopic physics. Graphene in its proper form is a strictly two-dimensional material—a single layer of carbon atoms arranged in a hexagonal crystal lattice. As a material graphene has exceptional electronic properties that enable the study of quantum electrodynamical (QED) phenomena and spur suggestions of intriguing applications, such as gas sensors, hydrogen storage, nanoresonators and field-effect transistors. Some more ambitious views even place graphene as the basis of a new type of microelectronics—or nanoelectronics—where a complex circuitry is carved directly on a graphene sheet.

Graphene can be exfoliated mechanically from high-quality graphite using a piece of adhesive tape. Although this so-called "Scotch Tape" method sounds very rudimentary, it is a viable means to create thin graphite films and even single-layer graphene. Detecting and characterizing thin graphite films is a bit more complicated, since a graphene monolayer is not optically visible on most substrates. Furthermore, modern visualization techniques, such as atomic-force, scanning-tunneling, Raman and electron microscopies have such a low throughput that their use in locating graphene from a large area is not practical. However, graphene can be made visible to the eye if the substrate is first oxidized or spin-coated with a layer of suitable thickness and refractive index. In this case the increased optical path and the notable opacity of graphene cause a small contrast between the graphene sheet and the substrate. This optical contrast gives also quantitative information about the thickness of the sheet. The calibration between observed contrast and the number of graphene layers can then be obtained by using Raman and atomic-force microscopies on preselected samples, as well as theoretical calculations.

The electronic properties of graphene manifest themselves in interesting current-voltage characteristics. At a zero electrostatic potential the carrier concentration in undoped graphene is zero. Nevertheless, the conductivity of graphene at this so-called Dirac point is finite. Measurements at LTL have shown that for short and wide graphene sheets at low temperatures, i.e., when scattering is negligible, the conductivity reaches a minimum of 4e²/πh. For longer sheets and for higher temperatures the conductivity is enhanced due to the effects of disorder and scattering.

A cunning way to investigate a mesoscopic system is to probe its noise properties. This method can yield information which is not accessible through standard I–V measurements. Running a DC current through the sample generates shot noise in the system. This noise "signal" can then be amplified and analyzed. Extracting information from the noise fluctuations is a tricky business, where all external noise sources must be suppressed. Moreover, the measurement setup has to be designed to minimize other noise mechanisms, such as thermal noise and 1/f noise.

At LTL the noise measurements are carried out at high frequencies in order to eliminate 1/f noise. Thermal noise, on the other hand, is minimized by running the experiments at low temperatures in helium dewars or dilution cryostats (Fig. 2). Together, shot noise and conductivity measurements in graphene have shown that transport at zero carrier concentration occurs via tunneling of electrons between the leads. This behavior has been explained to some extent by the evanescent-wave theory, but a great deal of work is still needed on both theoretical and experimental side in order to understand the true nature of graphene and utilize its full potential in everyday applications.